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Abstract

We systematically study the morphological and optical characteristics of the large-area micro/nanostructures produced by femtosecond laser irradiation on GaAs, Si, and brass. The experimental results demonstrate that along with the increase of laser fluence, significant changes in the surface morphology can be observed, and the most prominent phenomenon is the enlarging of the feature size of formed structures. Interestingly, by the fourier analysis of the treated areas, a peculiar phenomenon can be revealed: as laser fluence increases, the spatial frequencies of the structures change following a specific law – the allowed main frequencies are discrete, and appear to be a sequence of 2f, f, f/2, f/4, and f/8 (f is the fundamental frequency corresponding to the near-subwavelength ripples). In our opinion, the new frequency components of f/2, f/4, and f/8 originate in the 2-order, 4-order, and 8-order grating coupling. The law can offer us new insights for the evolving mechanisms of a variety of laser-induced micro/nanostructures in different scales. Furthermore, the optical characteristics of the treated surface are strongly dependent on the morphological characteristics that are mainly determined by laser fluence, such as the feature size of the micro/nanostructures, the topology of the surface morphology, the surface roughness, and the irregular degree of the formed structures. In general, as laser fluence increases in a moderate range, the specular reflectance of the structured surface would be significantly reduced. However, if laser fluence is excessive, the anti-specular-reflection effect would be much weakened. In ideal laser fluence, the micro/nanostructures produced by the near-infrared laser can achieve an ultra-low specular reflectance in the visible and near-infrared spectral region, which exhibits an attracting application prospect in the field of utilizing solar energy.

Figures (10)

The experimental results of the micro/nanostructures on GaAs produced by 1280-nm fs laser of linear polarization with different fluences. The laser fluence for area (a), (b), (c), (d) and (e) is 0.09, 0.11, 0.15, 0.19 and 0.25J/cm2, respectively. From the following processing parameters: the scanning velocity of 125 μm/s, the scanning interval of 8 μm, the pulse repetition frequency of 1 kHz, and the focal spot size of 36 μm, the average number of laser pulses per spot can be calculated to be about 1000 pulses. The sub-figures of left column are the OM images of the 400 × 400-μm2 treated areas taken under the same condition as spectral measurement. The sub-figures of the middle two columns are SEM images of the treated areas with different magnification factors. In (a) the double-headed arrow indicates the direction of laser polarization (in (b) to (e) the polarization directions are the same as in (a)). The sub-figures of the right column are the FFT images of the corresponding SEM images, which transform from the SEM images with small magnification factor except (a) and rescale to have the same spatial frequency scale.

The relative specular reflectances (Rr) of the treated areas on GaAs fabricated by 1280-nm fs laser in various fluence conditions as a function of laser wavelength (λ). Here curves a to e correspond to areas (a) to (e) in Fig. 2, respectively. The dashed line represents the reference spectrum of the untreated surface that is always equal to 1.

The experimental results of the micro/nanostructures on Si produced by 1280-nm fs laser of linear polarization with different fluences. The laser fluence for area (a), (b), (c), (d), (e), (f), and (g) is 0.16, 0.20, 0.26, 0.32, 0.40, 0.48, and 0.60 J/cm2, respectively. From the following processing parameters: the scanning velocity of 125 μm/s, the scanning interval of 8 μm, the pulse repetition frequency of 1 kHz, and the focal spot size of 36 μm, the average number of laser pulses per spot can be calculated to be about 1000 pulses. The sub-figures of left column are the OM images of the 400 × 400-μm2 treated areas taken under the same condition as spectral measurement. The sub-figures of the middle two columns are SEM images of the treated areas with different magnification factors. In (a) the double-headed arrow indicates the direction of laser polarization (in (b) to (g) the polarization directions are the same as in (a)). The sub-figures of the right column are the FFT images of the corresponding SEM images, which transform from the SEM images with small magnification factor except (a) and rescale to have the same spatial frequency scale for ease of comparison.

The relative specular reflectances (Rr) of the treated areas on Si fabricated by 1280-nm fs laser in various fluence conditions as a function of laser wavelength (λ). Here curves a to g correspond to areas (a) to (g) of Fig. 5, respectively. The dashed line represents the reference spectrum of the untreated surface that is always equal to 1.

The experimental results of the micro/nanostructures on brass produced by 800-nm fs laser of linear polarization with different fluences. The laser fluence for area (a), (b), (c), (d), (e), (f), (g), and (h) is 0.36, 0.45, 0.52, 0.60, 0.68, 0.82, 1.06, and 1.39 J/cm2, respectively. From the following processing parameters: the scanning velocity of 125 μm/s, the scanning interval of 8 μm, the pulse repetition frequency of 1 kHz, and the focal spot size of 45 μm, the average number of laser pulses per spot can be calculated to be about 1600 pulses. The sub-figures of left column are the OM images of the 200 × 200-μm2 treated areas taken under the same condition as spectral measurement. The sub-figures of the middle two columns are SEM images of the treated areas with different magnification factors. In (a) the double-headed arrow indicates the direction of laser polarization (in (b) to (h) the polarization directions are the same as in (a)). The sub-figures of the right column are the FFT images of the corresponding SEM images, which transform from the SEM images with small magnification factor and rescale to have the same spatial frequency scale for ease of comparison. The frequency components in the direction perpendicular to ripple wave vector are shown in the top right corner of the corresponding FFT images (the images have been rotated 90° for ease of comparison, and some of the color scales of the images have been reset in order to highlight the perpendicular frequency components).

The relative specular reflectances (Rr) of the treated areas on brass fabricated by 800-nm fs laser in various fluence conditions as a function of laser wavelength (λ). Here curves a to h correspond to areas (a) to (h) of Fig. 8, respectively. The dashed line represents the reference spectrum of the untreated surface that is always equal to 1.